In an optical semiconductor device, it has been studied to mount a plurality of light emitters on a surface of a substrate and to allow these light emitters to emit light, thereby using them as a planar light emitting source. In particular, it has been studied to use light-emitting diode bare chips (LED bare chips) as the light emitters. As such a light emitting source, there is, for example, one containing a substrate, a plurality of LED bare chips mounted on a surface of the substrate, a resin body for individually encapsulating these LED bare chips, a reflecting plate which has a reflecting hole opened corresponding to the resin body and a back side of which is adhered to the surface of the substrate, and a lens plate covering the whole of the substrate and having lenses in portions corresponding to the LED bare chips (see, JP-A-2005-223216). Accordingly, a step of encapsulating an optical semiconductor element and a step of mounting a metal layer as a reflecting plate on a substrate have hitherto been performed separately.